This invention relates to vesiculated silica microspheres which are useful as opacifying agents for coating compositions such as latex paints and especially useful as opacifiers and fillers for fibrous substrates such as paper.
A wide variety of porous silica particles are known in the art. The porous silica particles are commonly designed for use as catalysts or catalyst supports and less commonly for use as molding powders and insulating materials. Porous silica particles used as catalysts or catalyst supports are generally relatively large spherical particles, referred to as "beads", containing typical geltype pores having a maximum diameter of less than about 0.01.mu. Porous silica particles of this type have been prepared by acidifying an aqueous solution of sodium silicate to form a silica gel, then agitating the gel in an oil at elevated temperatures to form silica "beads" or spherical silica particles, as described, for example, in U.S. Pat. No. 2,528,767. Molding powders composed of porous silica microspheres having a diameter in the range of 0.1.mu. to 50.mu. have been made by spray drying sols containing colloidal silica, as described in U.S. Pat. No.3,301,635. The microspheres produced by this process are nonuniform aggregates of colloidal silica particles having very small pores, i.e., less than 0.01.mu. in diameter. The small pores present in the silica particles described above are too small for effective light scattering and render such silica particles relatively useless as opacifying agents.
Hollow silica spheres having an average diameter from 100.mu. to 2500.mu., useful as insulating materials, have been made by suspending particles of alkali metal silicate and a metal oxide with a compound which liberates a gas at elevated temperatures and heating the particles to 1000.degree.-2500.degree. F. (540.degree.-1371.degree. C.). The hollow silica spheres produced by this process are generally clear and transparent, rather than opaque, because the large air cell within the hollow sphere is too large to scatter light effectively.
A method for producing reticulated inorganic microparticles useful as pigment is described in U.S. Pat. No. 3,848,059. This process comprises mixing two separate water-in-oil emulsions, each containing a separate inorganic salt to form spherical shaped microparticles of an insoluble salt having an average particle size which falls within the range of 0.1.mu. to 5.0.mu.. The degree of porosity for these spherical microparticles is insufficient for use as highly effective opacifying agents.
Generally, the porous silica microparticles formed by the processes of the prior art are either large hollow microspheres or very dense microparticles which lack sufficient opacity for use in many applications.
The vesiculated silica microspheres of the invention have the size and porosity needed for high performance opacifiers and fillers.